Method for contrast matching of multiple images of the same object or scene to a common reference image

ABSTRACT

A method of imaging and a system therefore are provided. The imaging system includes an image forming device for generating a first image and a second image and a controller coupled to the image forming device. The controller receives the first image and the second image. In the method the controller generates an image ratio of the first image and the second image, regularizes the image ratio of the second image with respect to the first image to form a regularized image ratio and filters the image ratio to form a filtered ratio. The controller then multiplies the second image by the filtered ratio to form an adjusted image.

BACKGROUND OF INVENTION

[0001] The present invention relates generally to image systems and,more particularly, to matching the contrast of multiple images from theimage system.

[0002] Many types of digital imaging systems are known. In the medicalfield, such systems may include CT systems, X-ray system and MRIsystems. In each case multiple digital images may be formed of the samescene or object. The multiple images may be generated using the sameinput with different parameter sets. In many circumstances there existsa need to evaluate which of these images are optimal so that theappropriate parameters can be obtained. However, the problem with suchimages is that the brightness and contrast are different. Thus, theimages have to be mentally normalized. That is, brightness and contrastdifferences must be overlooked by the evaluator. This kind ofnormalization may lead to subjective bias and takes the mind of theevaluator away from the parameter evaluation.

[0003] Image processing algorithms are available in which differentparameter choices produce different looks. For example, one set ofparameters yields improved smoothness but produces artificially brightundesirable regions. The other set of parameters produces noisy imagesbut without bright regions. Adjusting each image individually is timeconsuming and may yield inconsistent results.

[0004] It would be desirable to match the brightness and contrast ofvarious types of images such as smooth images and noise images toproduce resultant images that are smooth but not artificially bright inone region. Also, there exists a need to match images of the same scenetaken at multiple time points such that they can be displayed with thesame brightness and contrast.

SUMMARY OF INVENTION

[0005] The present invention provides image processing that may be usedwith various types of imaging systems to reduce variability inbrightness and contrast between different images.

[0006] In one aspect of the invention, a method of contrast matching afirst image and a second image comprising: generating an image ratio ofthe first image and the second image; regularizing an image ratio of thesecond image with respect to the first image to form a regularized imageratio; filtering the image ratio to form a filtered ratio; andmultiplying the second image by the filtered ratio to form an adjustedimage.

[0007] In a further aspect of the invention, an imaging system includesan image forming device for generating a first image and a second imageand a controller coupled to the image forming device. The controllerreceives the first image and the second image. The controller generatesan image ratio of the first image and the second image, regularizes theimage ratio of the second image with respect to the first image to forma regularized image ratio and filters the image ratio to form a filteredratio. The controller then multiplies the second image by the filteredratio to form an adjusted image.

[0008] One advantage of the invention is that the subjective nature ofviewing images having different contrast and brightness portions isreduced. Another advantage is that the process can be automated so thatonce a first or reference image is chosen a number of images can bematched to the reference image quickly.

[0009] Other aspects and advantages of the present invention will becomeapparent upon the following detailed description and appended claims,and upon reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 is a schematic illustration of an image system inaccordance with a preferred embodiment of the present invention.

[0011]FIG. 2 is a flow chart for image processing according to thepresent invention.

DETAILED DESCRIPTION

[0012] While the following description is provided with respect to anX-ray device, the present application may be used with various types ofimaging systems including both medical and non-medical related fields.In the medical field, the present invention may be incorporated into butnot limited to a CT system, an MRI system system, and an ultrasoundsystem.

[0013] Referring to FIG. 1, an imaging system 10 in accordance with thepresent invention is shown. The imaging system 10 preferably includes ahousing 12 containing an x-ray source 14 or other type of imagegenerating source. The housing 12 may be a gantry having the ability formovement in multiple directions. The x-ray source 14 projects a beam ofx-rays 16 towards a detection array 18, which may also be containedwithin the housing 12. Positioned in between the x-ray source 14 and thedetection array 18 is a table 22, preferably not within housing 12, forholding an object 24 to be imaged by the imaging system 10. A dataacquisition system (DAS) 26 registers signals from the detection array18 and sends the information to a computer controller 28 for imageprocessing. Controller 28 is preferably a microprocessor-based personalcomputer. A control mechanism 29 may be used to control the movement andposition of the system components as well as power and timing signals tothe x-ray source 14.

[0014] The imaging system 10 may also include a monitor 30 and storagemedium 32 for viewing and storing information. While electronic andcontrol mechanism are illustrated, they are not required to perform theimaging techniques described herein and are merely being shown forillustration purposes only.

[0015] Although such a system describes generically an imaging system,the present invention preferably utilizes a high-resolution imager. Theimager has a pixel location and dimension of a high order of magnitudeprecision. Thus, each image will have multiple pixels in the image thatwill be covered by the shadow of the object. These multiple pixels canthen be mathematically evaluated to calculate either a size or positionthat has a degree of precision that is a small fraction of the dimensionof any one pixel. High-resolution imagers are well known in the priorart.

[0016] The detection array 18, on such high-resolution systems, includesa plurality of pixel panels 19, although a variety of pixel panel 19shapes, sizes and densities are contemplated. In addition, it isrequired that variations in pixel size and location be minimized. Avariety of detection arrays 18 includes a glass substrate 34, aphotodetector array 36 and a scintillator 38. In other embodiments,however, alternative detection array 18 configurations are contemplated.

[0017] Referring now to FIG. 2, the imaging processing is described. Instep 50, images that are desired to be imaged matched are stored intothe system. This may be done at one time or over a period of time. Asmentioned above, this may be performed using various types of imagingdevices. The process described below pertains to two images. The sameprocess may be used for multiple images in a similar manner as will bedescribed below.

[0018] In this example, two images A1 and A2 of the same object or sceneare to be imaged matched A2 to A1. For every pixel of A1 and A2, thefollowing relation holds: A1=A2*(A1/A2). By differentiation of thelogarithm of the above equation, the contrast function C(.) at a givenlocation is denoted by: C(A1)=C(A2)+C(A1/A2). As will be furtherdescribed below, the image division A1/A2 may optionally be regularizedrelative to the image to be matched A1 in step 52 when the image qualityis not good, e.g., noisy. Various types of regularization may beperformed. Regularization will be further described below.

[0019] In order to satisfy C(A/1)=C(A2)in the above equation,C(A1/A2)=0. A well known way to decrease the contrast is to low passfilter the ratio A1/A2 is shown in step 54. Therefore in step 56,contrast matching output equation for the two images A1 and A2 is thus:A1 _(M2)=A2*LPF(A1/A2), where A1 is the contrast matched version of A2with respect to A1 and LPF(.) is a low pass filter function. The lowpass filter function is further described below.

[0020] For multiple (N) images, let A1, A2, . . . AK . . . AN be the Nimages under consideration (K<N) and each of these images are to bematched to the same reference image A1. By extending the above logic toany of N images, say image K, the general relationship exists: A1_(MK)=AK*LPF(A1/AK), where A1 _(MK) is the contrast matched version ofAK with respect to A1. Thus, a generalized contrast matching has beenachieved since, C(A1)=C(A1 _(M2))= . . . =C(A1 _(MK))= . . . =C(A1_(MN)).

[0021] The choice of parameters in the low pass filter functionessentially determines the scale of contrast matching obtained. Varioustypes of low pass filters may be used. For example, a boxcar filter witha single parameter may be used. A boxcar filter smoothes an image by theaverage of a given neighborhood of pixels. It is separable and efficientmethods exist for its computation. Each point in the image requires justfour arithmetic operations, irrespective of the kernel size. The lengthof the separable kernel is variable and depends on the scale of contrastmatching desired. For example, if the kernel size is about one tenth ofthe image size, assuming a square image and a square kernel, excellentglobal contrast matching of images is obtained. On the other hand, usingtoo small a kernel size produces undesirable blobby patterns in thematched images. Therefore, a reasonably large kernel should be used toavoid any perceptible artifacts using this method.

[0022] To summarize, an image A2 has to be matched to another image A1of the same scene/objects to obtain the matched image A1 _(M2) using therelation: A1 _(M2)=A2*LPF (A1/A2), where LPF is a low pass filterfunction. Preferably the low pass filter function is a boxcar filter andthe parameters of the filter are application specific. For generalapplications, the filter kernel length is one-tenth the length of theimage (assuming a square image and square kernel). Furthermore, inpractice, the above equation may need to be modified in order to avoidnoise amplification during image division. Regularization may beperformed in a number of methods to prevent noise amplification duringimage division. The image division ratio has a numerator A1 and adenominator A2. One method to regularize image division is to add asmall constant to the denominator, i.e., denominator becomes (A2+ε)where as an example, ε=1.0. Thus the equation becomes: A1_(M2)=A2*LPF(A1/(A2+ε)).

[0023] Of course, if no regularization is to be performed, ε would be 0.

[0024] Another method for regularization is to replace the ratio (A1/A2)by a regularized ratio given by (A1*A2/(A2*A2+δ)), where as an example,δ=1.0. Thus the equation becomes: A1 _(M2)=A2*LPF(A1*A2/(A2*A2+δ)).

[0025] When a number of images A2, . . . , AK . . . , AN have to bematched to a single image A1, the above process may be performed in apair wise fashion to obtain A1 _(M2,), . . . A1 _(MK) . . . , A1 _(MN).

[0026] While the invention has been described in connection with one ormore embodiments, it should be understood that the invention is notlimited to those embodiments. On the contrary, the invention is intendedto cover all alternatives, modifications, and equivalents, as may beincluded within the spirit and scope of the appended claims.

1. A method of contrast matching a first image and a second imagecomprising: generating an image ratio of the first image and the secondimage; filtering the image ratio to form a filtered ratio; andmultiplying the second image by the filtered ratio to form an adjustedimage.
 2. A method as recited in claim 1 wherein filtering comprises lowpass filtering.
 3. A method as recited in claim 2 wherein low passfiltering comprises boxcar filtering.
 4. A method as recited in claim 1wherein forming an image ratio comprises forming an image ratio having anumerator and a denominator and wherein regularizing comprises adding aconstant to the denominator.
 5. A method as recited in claim I whereinforming an image ratio comprises forming an image ratio having anumerator and a denominator and wherein regularizing comprisesmultiplying the numerator by the second image and the denominator by thesecond image and adding a constant to the denominator.
 6. A method asrecited in claim 1 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is contrast matched to the first image.
 7. A method asrecited in claim 1 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is brightness matched to the first image.
 8. A method asrecited in claim 1 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is contrast and brightness matched to the first image. 9.A method as recited in claim 1 further comprising prior to filtering,regularizing an image ratio of the second image with respect to thefirst image to form a regularized image ratio.
 10. A method of operatinga digital image device comprising: generating a first digital image;generating a second digital image; matching the second image to thefirst image by, generating an image ratio of the first image and thesecond image; regularizing an image ratio of the second image withrespect to the first image to form a regularized image ratio; filteringthe regularized image ratio to form a filtered ratio; and multiplyingthe second image by the filtered ratio to form an adjusted image.
 11. Amethod as recited in claim 10 wherein filtering comprises low passfiltering.
 12. A method as recited in claim 11 wherein low passfiltering comprises boxcar filtering.
 13. A method as recited in claim10 wherein forming an image ratio comprises forming an image ratiohaving a numerator and a denominator and wherein regularizing comprisesadding a constant to the denominator.
 14. A method as recited in claim10 wherein forming an image ratio comprises forming an image ratiohaving a numerator and a denominator and wherein regularizing comprisesmultiplying the numerator by the second image and the denominator by thesecond image and adding a constant to the denominator.
 15. A method asrecited in claim 10 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is contrast matched to the first image.
 16. A method asrecited in claim 10 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is brightness matched to the first image.
 17. A method asrecited in claim 10 wherein multiplying comprises multiplying the secondimage by the filtered ratio to form the adjusted image where theadjusted image is contrast and brightness matched to the first image.18. An imaging system comprising: an image forming device for generatinga first image and a second image; and a controller coupled to said imageforming device for receiving said first image and said second image;said controller generating an image ratio of the first image and thesecond image, regularizing an image ratio of the second image withrespect to the first image to form a regularized image ratio, filteringthe regularized image ratio to form a filtered ratio, and multiplyingthe second image by the filtered ratio to form an adjusted image.
 19. Animaging system as recited in claim 18 further comprising a displaycoupled to said controller for displaying said adjusted image.
 20. Animaging system as recited in claim 18 further comprising a storagemedium for storing the first image the second image and the adjustedimage.